N,N'-dialkylphenylenediamines which are prepared by the reductive alkylation of a nitroaniline and a ketone in the presence of hydrogen and a suitable hydrogenation catalyst are utilized as additives for petroleum products such as gasoline in which the compounds act as an antioxidant and an inhibitor sweetening agent. The reaction for preparing the desired diamines may be effected under high pressure when utilizing either a batch or continuous type of operation. For example, when utilizing a batch type process the solid nitroaniline such as p-nitroaniline is poured into the reactor along with the catalyst, following which the ketone and the solvent are then added, the reactor is sealed, pressured with hydrogen, and heated to the desired operating temperature while subjecting the mixture to continuous agitation such as by stirring. The resultant product, which may comprise either a liquid or a low melting solid, is filtered free from the catalyst following which the solvent may then be removed by distillation.
When effecting the reductive alkylation process in a continuous manner, the reactants must, of necessity, be in solution in order that they may be continuously pumped into a reactor containing the catalyst and heated under hydrogen pressure. However, some difficulty is encountered when utilizing a continuous process inasmuch as p-nitroaniline has a poor solubility in most organic solvents. For example, it is almost insoluble in aliphatic hydrocarbons, only slightly soluble in aromatic hydrocarbons, and has a limited solubility in esters, alcohols and ketones. When utilizing a high molecular weight ketone as the alkylating agent, it has been found that excess ketone must be used as both the alkylating agent and solvent. However, the ketones which are used possess a particular disadvantage of being reduced to the corresponding alcohols and therefore it is necessary to dehydrogenate the alcohols back to the ketone. Likewise, when using low molecular weight ketones as alkylating agents, the ketones cannot be used in excess as a solvent due to the fact that they possess the facility of replacing both amine hydrogens of an aromatic amine, thereby forming undesirable overalkylated products such as trialkylated and tetraalkylated compounds.
The preparation of N,N'-diisopropyl-p-phenylenediamine, a particularly effective gasoline antioxidant and inhibitor sweetening agent, may be ued as an illustration of the problems which are encountered when preparing an N,N'-dialkyl-p-phenylenediamine. This compound is prepared by the reductive alkylation of p-nitroaniline with acetone. Inasmuch as, as hereinbefore set forth, p-nitroaniline as limited solubility in ketones, it is necessary to use an 8:1 mole ratio of acetone to p-nitroaniline in order to obtain a solution in which the p-nitroaniline is sufficiently solubilized so that the solution is capable of being pumped into a continuous plant. However, stoichiometrically, only a 2:1 mole ratio is required. The use of such an excess of acetone will cause further reaction with the dialkyl product to form unacceptable amounts of tri-N-alkylated by-products and will result in the loss of both acetone and hydrogen in the reduction of acetone to alcohol. In addition, the excess acetone will also interfere with the separation of water in the solvent recovery system of the plant.
Some prior art references have shown reductive alkylation processes. For example, Canadian Patent No. 862,797 discloses a reductive alkylation process in which a sulfided platinum catalyst is used for the reductive alkylation of an organic compound containing an amino and/or a nitro substituent. Likewise, U.S. Pat. No. 2,969,394 relates to a novel combination process which includes the reductive alkylation of an aromatic amino or nitro compound with a ketone during which an alcohol is formed from the ketone and converted back to the ketone for further use within the process. The patent describes an integrated continuous process for the manufacture of N,N'-di-sec-butyl-p-phenylenediamine in which the by-product, namely, 2-butanol, which results from the reduction of an excess of methyl ethyl ketone, which is employed to dissolve the p-nitroaniline, is isolated and converted back to the ketone for recycle. As will be hereinafter shown in greater detail, in the process of the present invention the use of excess ketone is avoided by solubilizing the nitroaniline in a solvent of the type set forth and therefore utilizing only enough ketone to satisfy the stoichiometry of the reductive alkylation reaction.
Another prior art patent, namely, U.S. Pat. No. 3,522,309 teaches the use of polar solvents such as a 5-20% amount of lower alcohols or hydrogenated hydrocarbons to increase the rate of the reductive alkylation reaction. However, the solvents which were utilized in this reference are not particularly effective as solvents for the nitroanilines.
As hereinafter set forth in greater detail, it has now been discovered that by utilizing certain organic solvents of the ether type it is possible to reductively alkylate a nitroaniline with ketones, and particularly low molecular weight ketones, in which the alkylating agent is present in a relatively low mole ratio, the ethers acting as a solvent for the nitroanilines.